Hydrochloric acid (HCl)-based leaching of calcium-based rock, with the goal of extracting valuable metals such as rare earth elements, can produce a CaCl2 solution by-product. The extraction of these metals relies on the use of concentrated HCl, typically of azeotropic strength. The CaCl2 solutions cannot be released into the environment due to their high concentration of chloride ions and have to be processed further. This is ideally done in a way that offers the possibility to reclaim the HCl used by the process (acid recycling). Therefore, in order to enable the leaching of minerals with HCl on an industrial scale, the acid needs to be recovered. Conventional HCl recovery techniques, such as pyrohydrolysis for iron chloride solutions, are used at industrial scale, but are not economically feasible for the treatment of CaCl2 solution for fundamental thermodynamic reasons. The pyrohydrolysis of CaCl2 requires temperatures of up to 1000° C. to recover the HCl.
It is thus desirable to have a means to treat the concentrated calcium chloride solutions generated from the processing of calcium-based rock, thereby allowing recovery of the chloride units as high strength HCl for re-use.